2 Answers
2

Check out app.js in the ./build folder. (Run $ "truffle build" if it isn't there.)

Find lines with "web3" ... in particular = require("web3");

Truffle merges in a lot of stuff so we can be less concerned about implementation details. The built version will have a lot of unfamiliar looking stuff, but you should see familiar lines from your app near the end after /// END TRUFFLE BOOTSTRAP

I believe (though I can't find a reference for this) Truffle automatically injects web3 during truffle serve. Theoretically, a dapp browser or a plugin like MetaMask would also inject web3, which is why you wouldn't see any reference to it--the user probably already has it.

If you want to manually include web3, you can get it here (or through npm). Here is my answer elsewhere on how to set up a functioning web3 manually.

Awesome, thanks! Just a question; is truffle now the most used development framework in ethereum land, or can you recommend others which are also good?
– kramer65Jan 5 '17 at 11:46

I wouldn't be surprised if truffle is the most popular, but I personally develop without a framework. It's ultimately personal choice--the field is so new that there's not much vendor buy-in, anyway.
– Matthew SchmidtJan 5 '17 at 18:50